libkmod: refactor builtin module handling

Remove arbitrary limits due to file sizes (INTPR_MAX check). Reduce
amount of system calls by utilizing stream functions.

Also make sure that no TOCTOU could ever happen by not iterating
through the file twice: First to figure out amount of strings, then
parsing them. If the file changes in between, this can lead to
memory corruption.

Improves overall performance as well.

Signed-off-by: Tobias Stoeckmann <[email protected]>